Unexpected significance of a minor reaction pathway in daytime formation of biogenic highly oxygenated organic compounds

Secondary organic aerosol (SOA), formed by oxidation of volatile organic compounds, substantially influence air quality and climate. Highly oxygenated organic molecules (HOMs), particularly those formed from biogenic monoterpenes, contribute a large fraction of SOA. During daytime, hydroxyl radicals initiate monoterpene oxidation, mainly by hydroxyl addition to monoterpene double bonds. Naturally, related HOM formation mechanisms should be induced by that reaction route, too. However, for α-pinene, the most abundant atmospheric monoterpene, we find a previously unidentified competitive pathway under atmospherically relevant conditions: HOM formation is predominately induced via hydrogen abstraction by hydroxyl radicals, a generally minor reaction pathway. We show by observations and theoretical calculations that hydrogen abstraction followed by formation and rearrangement of alkoxy radicals is a prerequisite for fast daytime HOM formation. Our analysis provides an accurate mechanism and yield, demonstrating that minor reaction pathways can become major, here for SOA formation and growth and related impacts on air quality and climate.


Section S2. Instrumentation & experimental overview
For gas-phase species, a set of instruments was applied, including a laser-induced fluorescence system (79) to measure concentrations of OH, HO2•, and RO2•, a proton-transfer-reaction time-offlight mass spectrometer (PTR-ToF-MS, Ionicon Analytik, Austria) for concentrations of α-pinene, as well as a NOX analyzer (Eco Physcis TR480) and an O3 analyzer (Ansyco, model O341M). Their concentrations within the first 15 min reaction time are shown in Fig. S3. Note that concentrations of O3 are under the detection limit (detection limit of 1 ppb and a precision of 0.5 ppb) in both low and high NO experiment and thus cannot be seen in the figure. This low O3 concentrations also ensure that oxidation by OH radicals is overwhelming in our experiments. Further note that in the low NO experiment, the NOX analyzer encountered a technical problem. The NO concentration based on the average concentration in the three other experiments at the same conditions were provided (solid black markers in Fig. S3A), which show good agreement with the NO concentrations (grey line) in Fig. S3A derived from the established methods in the SAPHIR chamber by Rohrer et al. (52). For all the low NO experiments in SAPHIR, a typical background concentration before roof opening is 10-30 ppt NO, which is also included in the Master Chemical Mechanism (MCM) HOM modeling for α-pinene + OH shown in this study. Measurements in the other low NO experiments (solid black markers) and simulated calculations (grey lines) in Fig. S3A showed that NO concentrations increased from 0-30 ppt to ~100 ppt during the first 15 min reaction time in this study.
For the particles, the number and size distribution were measured using a Scanning Mobility Particle Sizer (SMPS, TSI, DMA 3081/CPC 3785), with total number concentrations and surface area concentrations shown in Fig. S3. In both low and high NO experiments, negligible particles were formed during the first 15 min reaction time, with total number concentrations of <40 cm -3 at low NO and <20 cm -3 at high NO.
The physical parameters including temperature, pressure, relative humidity (RH), and solar irradiation (photolysis frequencies of J values such as JNO2, JO1D, JH2O2, and JHONO) were recorded throughout the chamber experimental runs.

Section S4. Uncertainties of HOM yield
The direct calibration method using HOM standards is not possible so far, mainly due to the difficulty to synthesize pure HOM and unknown chemical structures of many HOM. However, as discussed by Ehn et al. (11), the sensitivity of NO3 --CIMS to H2SO4 is similar to HOM, and thus the calibration coefficient of H2SO4 is commonly applied to HOM (11,12,21,78). Effective deprotonation of H2SO4 leads to formation of clusters with NO3ions, with rates close to the kinetic collision limit. In this study, most of detected HOM are binding with NO3reagent ions, also near the collision limit as H2SO4. Combining the theoretical calculations (39,40) and our previous experimental results (41) discussed here and in the main text, we conclude that the sensitivity of NO3 --CIMS to all HOM is similar to that of H2SO4 in this study and the calibration coefficient of H2SO4 can be applied to all HOM. If the HOM charge at rates lower than the collision limit, the calibration coefficient would be overestimated and the HOM concentrations derived using the calibration factor of H2SO4 thus constitute a lower limit with an uncertainty by a factor of 2 (58).
The peak intensities of H2SO4 can be converted to concentrations following Eq. S5.
Moreover, HOM concentrations were corrected for wall loss and dilution. The value of 6×10 -4 s -1 and 2.2×10 -3 s -1 was respectively used for the wall loss correction kwall loss at low and high NO based on our previous study (38) and our recent updates for different conditions. Acting mixing due to a running fan in our high NO experiments leads to a larger wall loss rate. Although wall loss rates of less oxygenated compounds can change during an experiment (81), in this study, we focus on HOM and short reaction times (0-15 min), therefore HOM yields are not expected to be sensitive to the wall loss rate. Sensitivity analyses showed that variation of the wall loss rate from +100% to -50% led to changes in the molar HOM yield between +28.2% and -13.9% in the low NO experiment and between +76.0% and -36.2% in the high NO experiment. The dilution rate kdilution loss was derived from a constant dilution flow rate of the chamber, leading to a value of 1.5×10 -5 s -1 .

Section S6. C 7 HOM
Besides C10-HOM, both C<10-HOM and C>10-HOM were observed, as shown Fig. S9. As discussed in the main text, C>10-HOM observations (with C20-HOM as an example) also indicated a chemistry dominated by C10H15OX•. For C<10-HOM, C7-HOM are the most abundant products in both low and high NO cases. Their detailed formation chemistry of C7-HOM and their relation to C10-HOM is discussed below. Two families of peroxy radicals, C7H9OX• and C7H11OX•, are identified in C7-HOM (Fig. S9), together with their corresponding C7 closed-shell products, carbonyls (C7H8OX and C7H10OX) and organic nitrates (C7H9NOX and C7H11NOX). Higher concentrations of C7H9OX•-related C7 closedshell products were observed compared to C7H11OX•, with concentration ratios of carbonyls (C7H8OX/C7H10OX) of 9.8 and 7.1 at low and high NO, respectively, and concentration ratios of organic nitrates (C7H9NOX/C7H11NOX) of 5.7 and 8.8 at low and high NO. According to the current master chemical mechanism MCM v.3.3.1 (48,83) and calculations by Vereecken and Peeters (29), C7H11OX• are expected to be formed from the decomposition of C10H17OX• with acetone as a byproduct as shown in Fig. S10A. C7H9OX• are proposed to be produced from decomposition of C10H15OX•. As shown in Fig. S10, one alkoxy step for R6OO, R9OO, and R10OO ( Fig. 2 in the main text) leads to alkoxy radicals R6O, R9O, and R10O, whose subsequent decomposition reaction is expected to form alkyl radicals C7H9O2,4• and acetone. The following autoxidation of successive O2-addition and H-shift is proposed to form highly oxygenated C7H9OX•. Thereby, the C7-HOM product distribution is consistent with C10-HOM, further confirming the importance of C10H15OX• in α-pinene + OH. Moreover, higher concentrations of C7-HOM at high NO ( Fig. S9B) than those at low NO ( Fig.  S9A) are also consistent with more active alkoxy steps at high NO than at low NO, where unimolecular reactions of C10-RO2• are more competitive due to the longer lifetime at low NO. Overall, the distributions of C7-HOM at both low and high NO are consistent with the important role of alkoxy radicals in HOM formation, particularly for C10H15OX• and its related HOM in this study.
Similar to the C10-HOM case, more C7H9OX•-related HOM (carbonyls C7H8OX and organic nitrates C7H9NOX) are formed at earlier reaction time in high NO experiments compared to low NO experiments, as shown in Fig. S11. As proposed, C7H9OX• and C7H11OX• are respectively formed from C10H15OX• and C10H17OX•. As discussed in the main text, the formation rate of C10H15OX• depends on NO concentrations, which does not influence the formation of C10H17OX•. Concentration ratios of their related C7 closed-shell products (C7H8OX/C7H10OX and C7H9NOX/C7H11NOX) reached values of 1, where C7H9OX• chemistry is considered to be equivalent to C7H11OX, at a reaction time of >5 min at low NO and 1-2 min at high NO. This faster appearance of C7H9OX•-dominated chemistry at high NO again agrees with the temporal behavior of C10 HOM (Fig. 3 in the main text) and provides additional evidence for the importance of alkoxy steps in this study. Additionally, the formation of C7 HOM is an evident support for the existence of alkoxy steps in α-pinene + OH. All data are averaged to a time resolution of 1 min. In each panel, error bars of C7 carbonyls and organic nitrates represent one standard deviation (left axes, peak intensities lower than background signals are not shown) and error bars of concentration ratios (right axes, uncertainties more than 200% are not shown) were calculated using error propagation.

Section S7. Pinonaldehyde production from OH addition channel
For the established OH addition channel, direct autoxidation is expected to occur in channel A in Fig. S12, leading to C10H17OX• (25)(26)(27). The branching ratio for this ring-opened RO2• ranges from 7.5% to 22% (MCM v3.3.1 and Peeters et al. (28), respectively). This limited reaction contribution can be one possible reason for the low concentrations of C10H17OX• related HOM observed in this study. Channels B and C lead to a major product pinonaldehyde C10H16O2 (84), whose aldehydic hydrogen atom can be readily abstracted by OH radical again, leading to the indirect formation of C10H15OX• via following autoxidation steps (Fig. S12B, S12C). The limited role of hydrogen abstraction from pinonaldehyde for production of C10H15OX• related HOM in this study will be discussed in the following.  (25)(26)(27). The formation of (B, C) C10H15OX• from hydrogen abstraction from pinonaldehyde, the main product after OH addition to α-pinene. Note that autoxidation pathways in channel (A) likely play a more important role at low NO than alkoxy steps (Fig. S10A) which is expected to be more important at high NO.
Compared to the direct hydrogen abstraction from α-pinene, the hydrogen abstraction from pinonaldehyde pathway is expected be occur later since pinonaldehyde must first be formed via OH addition to α-pinene (85). Considering the rapid formation of C10H15OX• and related HOM (within 3 min) observed in this study, first-generation peroxy radicals are likely to dominate in C10H15OX• formation compared to any second generation peroxy radicals from pinonaldehyde. In order to quantify the relative importance of the two pathways, we calculated the relative reaction rate (Equation S8) of hydrogen abstraction from α-pinene to that of pinonaldehyde by OH radicals, as shown below: (-0%/+160% at low NO and -0%/+50% at high NO) using error propagation.
The relative reaction rate along with uncertainties during the first 15 min reaction are shown in Fig. S13. Hydrogen abstraction from α-pinene is more than 70 times faster than that from pinonaldehyde at low NO and more than 8 times at high NO. Note that the concentrations of pinonaldehyde were estimated from consumed α-pinene and yields of pinonaldehyde, thus reflecting only the production. As pinonaldehyde is continuously consumed by OH, its true concentration should be lower, and thus its relative importance is even overestimated using this method. Overall, we thus conclude that pinonaldehyde has only a minor to negligible contribution to HOM formation at early stages of the experiments.

Section S8. Theoretical methodology
The geometries of the intermediates and transition states in the mechanism were first optimized using the M06-2X/cc-pVDZ methodology (88,89), with an exhaustive characterization of all conformers for each reactant and transition state. All geometries obtained thus were further optimized at the M06-2X-D3/aug-cc-pVTZ level of theory which includes Grimme et al. (90). D3 diffusion corrections (90,91). Moments of inertia for molecular rotation, and wavenumbers for vibration were obtained at the same level of theory, with a vibrational scaling factor of 0.971 (89,92). The transition states were validated by IRC calculations and by visual verification of the imaginary frequency vibrational mode. Finally, the relative energies were further improved by single-point calculations at the CCSD(T)/aug-cc-pVTZ level of theory (93,94); this final level of theory is referred to as CCSD(T)//M06-2X-D3. T1 diagnostics at that level of theory did not reveal high multi-reference character for any of the structures (T1 ≤ 0.028). The expected uncertainty on the reaction barrier heights at this level of theory is ±0.5 kcal mol -1 . All quantum chemical calculations were performed using the Gaussian-16 software suite (95). The quantum chemical data underlying the theoretical kinetic calculations can be found as a text file at https://doi.org/10.26165/JUELICH-DATA/GLU9BW.
The rate coefficients for the individual reactions at the high-pressure limit were then calculated using multi-conformer transition state theory, MC-TST (96), incorporating the data for all conformers obtained as described above. Tunneling is accounted for using an asymmetric Eckart barrier correction (97,98). Based on earlier work at a similar level of theory in comparison with experimental data on H-migration in RO2• radicals with no or only one oxygenated functionality (50,99) and available theoretical literature data on ring closure reactions (56), we estimate the thermal rates to be accurate to a factor 2 to 3.

Section S9. Theoretical kinetic study of RO2• radicals after H-abstraction
As already studied in prior literature (28,29,33), the dominant H-abstraction sites in α-pinene are those that lead to allyl-resonance stabilized alkyl radicals. Under atmospheric conditions, these allyl radicals will undergo O2 addition, which can occur on either side of the ring for endocyclic radical sites. Formally, there are then 7 distinct RO2• radicals that need to be examined (see Fig.  S14). Of these, R2OO is expected to be dominant under atmospheric conditions, with an estimated 40:60 syn to anti ratio. For each of these RO2• radicals, we examined all geometrically accessible intramolecular Hmigrations; ring closure reactions leads to strained tricyclic structures and are either geometrically not feasible or energetically not competitive even without the additional ring strain of the bicyclic molecular frame (56). The calculated rate coefficients are tabulated below in Table S1. As can be seen, all unimolecular reactions are slow, and not competitive with bimolecular reactions under the experimental conditions, where RO2• lifetimes are of the order of 1 minute or less (≤10 2 s). Section S10. Theoretical kinetic study of RO2• radicals formed from R1OO and R2OO Due to the long unimolecular lifetime of the R1OO and R2OO peroxy radicals, they are likely to undergo bimolecular reactions, where reactions with other peroxy radicals or with NO will yield alkoxy radicals in significant fractions (100). The formation routes of RO2• radicals formed from R1OO and R2OO are shown in Fig. S15. Anti-R2O will readily open the 6-membered ring, k ~ 2×10 5 s -1 (28), forming a cyclobutyl radical that reacts with O2 under atmospheric conditions, forming syn-and anti-R4OO. Syn-R2O is expected to undergo a 1,5-H-migration, k ~ 3×10 6 s -1 (28) forming a chemically excited alkyl radical which, due to the ring strain in the 4-membered ring, is expected to undergo ring opening for a large fraction. The resulting cyclo-allyl radical will then add O2 on either side of the ring, forming syn-and anti-R5OO and syn-and anti-R14OO. The fate of R1O is not discussed in detail. We provide a partial reaction scheme in the main text, and in Fig. S15C, S15D. We have performed theoretical kinetic calculations on the fate of the R3OO, R4OO, R5OO and R14OO radicals, which can undergo H-migration reactions and ring closure reactions. The calculation results are tabulated in Table S2. As can be seen, only unimolecular reactions in syn-R14OO and the fast 1,8-H-migration in syn-R3OO (included in Fig. 2 in the main text) are competitive against bimolecular reactions under the current reaction conditions with RO2• lifetimes of less than 1 min. The majority of the peroxy radicals are thus expected to undergo bimolecular reactions, which for a significant fraction will yield the corresponding alkoxy radicals (R3O, R4O, R5O, and R14O, shown in Fig. S16). Their fate can be estimated using established structure-activity relationships and available theoretical work (51,101,102). The dominant loss process for R3O and R4O radicals will be the opening of the 4-membered ring, forming R6OO and R10OO molecules, respectively, after reaction with O2 (see Fig. S16). Similarly, R14O radicals will instantly open the 6-membered ring, forming an α-OH alkyl radical that reacts readily with O2 to form a ketone + HO2•, terminating the radical chain. For R5O radicals we estimate that the ring structure will hamper H-migration and ring closure reactions, leaving the favorable opening of the 6-membered ring as the dominant fate, forming R11OO after O2 addition.

Section S11. Procedure for constructing oxidation schemes after ring opening
Explicit theoretical calculations on the intermediates get increasingly costly as more oxygen atoms are added. At this time, we chose not to do such calculations for intermediates where both rings are opened. Instead, we anticipate that in the absence of ring strain the rate of reactions can be fairly well described based on structure-activity relationships (SARs).
For most H-migrations in RO2• radicals, we base ourselves on the SAR by Vereecken and Nozière (50); this SAR was reported to reproduce the scarce experimental data within a factor of 2, but for multi-functionalized species such as studied in this work the scatter on the data within each SAR category reaches an order of magnitude. For those reaction classes that are not covered by the SAR, we attempt to extrapolate the reactivity trends in that SAR. the expected uncertainty increases by at least an order of magnitude as no reliable data is available to validate the extrapolation. In most cases, however, this is sufficient to determine whether the reaction can contribute or not, as reaction rates typically differ by several orders of magnitude and the SAR covers the most common and favorable reaction classes explicitly. For H-migrations in cycloperoxides, we additionally rely on the systematic study by Vereecken et al. (56), who explicitly calculated rate coefficients for peroxy radicals formed after RO2• ring closure reactions. For ring closure reactions in unsaturated RO2• we rely on the SAR by Vereecken et al. (56). We assume that the rate of ring closure is not affected by the presence of another cycloperoxide ring; this approximation is unlikely to be accurate, but insufficient data is available to improve the estimates.
In assessing the fate of an RO2• radical with one or more -OOH groups, we account for the possibility of H-atom scrambling, as described extensively in the literature (53-55, 99, 103). The H-migrations between the -OO• and -OOH groups often allow for additional pathways, and are typically much faster than other unimolecular channels, such that the scrambling leads to a fast equilibration among all accessible OOH-substituted RO2• radicals, and the dominant channel is chosen among those available to the pool of RO2• radicals. We refer to Vereecken and Nozière (50) for a more detailed description of this feature.
In the Fig. S17 Fig. 2 in the main text) were determined and shown in Fig. S18.  Note that alkoxy steps are not included in the figure. All of the rate coefficients shown in this figure were derived from SARs, as described in Section S11.

Section S12. Incorporation of the hydrogen abstraction channel in MCM
The hydrogen abstraction channel is not included in the α-pinene + OH chemistry in the current MCM v3.3.1 (48). Based on the calculated reaction rates and mechanism in Fig. 2 in the main text, we added the hydrogen abstraction channel to the MCM (http://mcm.york.ac.uk/home.htt), with detailed reactions and rate coefficients listed in Table S3, which account for the site-and stereospecific data from the theoretical study described above. In the initial step of α-pinene + OH, a branching ratio of 0.09 is applied to the hydrogen abstraction channel (reactions 4-7 in Table S3) and the branching ratio of OH addition channel is also updated (reactions 1-3 in Table S3). The main loss of RO2• radical is assigned to bimolecular reactions with NO to produce organic nitrates (10%) and alkoxy radicals (90%). Bimolecular reactions of RO2• + HO2• are also included, but bimolecular reactions of RO2• + RO2• and OH elimination by unimolecular RO2• reactions are not included. Reactions for the hydrogen abstraction channel start at α-pinene (C10H16) + OH and include the formation of peroxy radicals containing up to ten oxygen atoms (C10H15O10•). Besides the RO2• autoxidation reactions shown in Fig. 2 in the main text and Fig. S18, all other reactions with rate coefficients more than 1×10 -3 s -1 are also included for completeness. While we do not have theoretical calculations for syn-R1OO/syn-R1O, which accounts for about 20% of the hydrogen abstraction channel, similar chemistry is assumed for syn-R1O to syn-R2O, shown as reactions of 33-34 and 49-50 in Table S3. Totally, 139 reactions were included for calculations, including peroxy radicals from R1OO to R29OO in this study.
The MCM simulations were calculated using an open access program iChamber (104). Production and photolysis of HONO were also incorporated into the calculations according to Rohrer et al. (52). Physical parameters, including JHONO, JNO2, temperature, and relative humidity, were constrained to measured data with a time resolution of 10 s. In both low and high NO experiments, initial concentrations of α-pinene were set as 20 ppb. Initial concentrations of NO were set as 0 ppb at low NO and 20 ppb at high NO. A reaction period of 15 min was calculated for both low NO and high NO using the above settings. Condensation sink of HOM to particles is negligible due to the absence of nucleation in this time frame. The wall loss rate of 6×10 -4 s -1 (low NO) and 2.2×10 -3 s -1 (high NO) were applied to HOM with six and more oxygen atoms, and a dilution rate of 1.5×10 -5 s -1 was applied to all compounds in our MCM calculations.
The closed-shell product C10H15NO9 is used to compare our calculated results to the observations (CIMS data). These C10H15NO9 species are formed from C10H15O8•. The rate coefficients of RO2• + NO  RONO2 are known, and organic nitrate formation is included in our calculations. Moreover, in both low and high NO experiments, the observed concentrations of C10H15NO9 are sufficiently high for comparison, with an abundance of 11.3% (low NO) and 27.0% (high NO) relative to the highest closed-shell products C10H14O10. In our calculations, C10H15NO9 are formed in reactions 115, 118, 999, 127, 130, and 139 as listed in Table S3. Their calculated time series and CIMS data at low and high NO are shown in Fig. S19. Our simulations agree within the uncertainties with the observations at both low and high NO. The formation of C10H15NO9 accelerates after around 5 min reaction time at low NO, while C10H15NO9 formation starts after about 2 min reaction time at high NO. Note that as mentioned in Section S2, some background NO concentration was present in the low NO experiments; thus a sensitivity test of initial NO concentrations of 0-30 ppt was also conducted for low NO experiment, with dashed lines shown in Fig. S19A. With each increase in the initial NO concentration, it takes less time to accelerate the formation of C10H15NO9. This is consistent with the expectation that higher NO concentrations promote the production of alkoxy radicals. As shown in Fig. S19A, all simulated C10H15NO9 (with an initial NO concentration from 0-30 ppt) are within the standard errors of the observations. The background NO concentrations have negligible influence on our experiments.
The alkoxy production rate was estimated based on the reaction of RO2• + NO  RO• + NO2 in this study, as shown in Fig. S20. Concentrations of RO2• and NO were derived from our MCM simulations and a uniform rate constant of kRO2+NO was used. The organic nitrate yield is assumed to be 0.1. Overall, a lower alkoxy production rate was observed in low NO than in high NO. Also, as expected, the formation of alkoxy radicals is slow at the beginning of the low NO experiment while immediate formation of alkoxy radicals is observed at high NO. This again emphasizes the requirement for ring opening, and NO dependence in the HOM formation from H-abstraction channel in α-pinene + OH reactions in this study.
We also examined the sensitivity of the model results to the model parameters. Two sets of the rate coefficients were applied, including one set derived from explicit calculations in this study (as discussed above), and the other set assigned as fixed values (50,56), as shown in Table S3. Calculated time series of C10H15NO9 using explicit and fixed rate coefficients are shown in Fig.  S19. Overall, no significant difference is observed in concentrations of C10H15NO9 using explicit and fixed values, and the delayed C10H15NO9 formation in low NO are both observed using explicit and fixed values. Hence, the model results are not overly sensitive to the rate coefficients used. However, without knowing reaction pathways and chemical structures, which are still derived from explicit calculations, the fixed values cannot be assigned. In this case, it is essential to determine the reaction pathways based on explicit calculations to simulate product formation in the MCM. Further work is needed to incorporate more detailed H-abstraction channel in MCM.
To further compare C10H15OX• and C10H17OX• chemistry in the HOM formation from OH oxidation of α-pinene, autoxidation pathway of C10H17OX• was added to the MCM calculations, with reactions in Table S4 following Xu et al. (27). In OH addition channel, autoxidation is expected to only occur in the ring-opened RO2• (C10H17O3•, Fig. S12A). Reactions after the ringopened RO2• (>1×10 -3 s -1 ), from C10H17O3• to C10H17O11•, were incorporated into the calculations, including Scheme 2, 3, and 5 in Xu et al. (27). Note that reactions leading to low oxygenated termination products (O<6), such as P2 in their Scheme 2, are not included to simplify the model. For rate coefficients or branching ratios missing in Xu et al. (27), uniform values were applied. For example, Scheme 5 in their paper, only anti-C10H17O3• can undergo subsequent rapid autoxidation. A stereospecific branching ratio of 50% was applied to α-pinene+OH, as shown reactions 1 and 2 in Table S4. The rate coefficients of RO2• H-shifts were estimated from Vereecken and Nozière (50). Rate coefficients of ring-closure, H scrambling, RO• decomposition, and RO• H-shift, were given as 1 s -1 , 1×10 2 s -1 , KDEC (1×10 6 s -1 ), and 1×10 5 s -1 according to empirical knowledge from literature (54)(55)(56). Organic nitrate C10H17NO8 was used to compare our calculated results to the observations, as shown in Fig. S21. Overall, our calculation results agree within the uncertainties with the observations at both low and high NO. Similar time series were observed at low NO and high NO and no delay was observed at low NO. This is consistent with independence of C10H17OX• formation pathway on NO.
As only part of the C10H15OX• (X≤10) and C10H17OX• (X≤11) chemistry was incorporated into the MCM calculations, C10H17NO8 and C10H15NO9 are used as an example to compare C10H15OX• and C10H17OX• chemistry in the HOM formation from OH oxidation of α-pinene. They are formed via similar formation pathway, i.e., two autoxidation steps and following bimolecular reactions with NO. Their concentration ratios from MCM calculations are shown in Fig. S22. At low NO, concentration ratios of C10H15NO9 to C10H17NO8 increased with reaction time. This is expected as a result of an accelerated C10H15OX• chemistry and C10H17OX• chemistry independent of NO as reaction proceeds, with increasing NO from HONO photolysis. Higher initial NO concentration leads to higher concentration ratio of C10H15NO9 to C10H17NO8, also supporting the acceleration of C10H15OX• chemistry by NO. At high NO, concentration ratio of C10H15NO9 to C10H17NO8 is relatively stable, with a quick rise at the start of the reaction and a slight decrease over later reaction times. As high NO concentrations lead to rapid alkoxy steps, C10H15OX• chemistry is expected to dominate over C10H17OX• chemistry and to lead to a relatively stable concentration ratio of C10H15NO9 to C10H17NO8. This time series pattern is similar to the observed time series of C10H15NOX and C10H17NOX (Fig. 3B, 3D), with an increasing concentration ratio of C10H15NOX to C10H17NOX at low NO and stabilization at low NO. However, it takes more time for observed concentration ratios (~5 min) to achieve stabilization than in MCM calculations (almost immediately after the reactions). This difference may be attributed to the incomplete mechanism of C10H15OX• chemistry, particularly for different chemistry of various isomers as only the most likely pathway is proposed in this study, and to incomplete C10H17OX• chemistry where only example pathways are available in the literature (27) and included in MCM. Part of the difference can also attribute to large uncertainties of observed concentration ratios (Fig. 3). Further studies are warranted for more detailed H-abstraction channel in OH oxidation of α-pinene.       Table S4. OH addition mechanism added to α-pinene + OH subset of MCM. Incorporated reactions and rate coefficients (following Xu et al. (27) in the default α-pinene + OH subset of MCM mechanism.

No. Reactions
Rate coefficients  Table S5. Reported products and their molar yields from α-pinene + OH and α-pinene + O3 using different ionizations. Note that molar yields that are not from direct measurements are marked with asterisks (*).